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Old 12th Jan 2012, 2:23 pm   #33
kalee20
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Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 7,061
Default Re: capacitors in series - pitfalls?

I've been following this and although I agree with the self-equalising of voltage in some types (due to internal leakage rising rapidly with voltage, a bit like a soggy zener diode, as has been mentioned), I wouldn't rely on it.

If capacitors are indeed mismatched (such as 100GΩ and 10GΩ) the the voltage will, in the absence of balance resistors, be distributed in the ratio 10:1.

If you add balance resistors, then far more leakage current will flow, because the 10GΩ capacitor will now have half the supply voltage across it so will leak more current. One could argue that it is being more stressed, current-wise. But, as long as it is still within its voltage rating, it will be designed to cope with it. However, the voltage across the other capacitor will be reduced from 91% of the supply to only 50%. And this reduction of voltage stress could be far more preferable.

Some capacitors, when overstressed voltage-wise, don't start progressively leaking more - they just fail (film/foil types particularly, but also solid tantalum). The internal stored energy causes thermal damage at the site of the breakdown - and if the capacitor is charged to a higher-than-rated voltage, the energy is a lot more than normal (energy being proportional to volts squared).

It's also worth bearing in mind that without balancing resistors, even perfectly matched capacitors could share voltage badly, due to external contamination, condensation, etc, precipitating a failure.
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